1. Field of the Invention
The present invention relates to an optical recording medium of a pitting recording type provided with metal recording layer in which recording marks are formed by irradiation with a violet laser beam of a wavelength in the range of about 380 to about 450 nm. The optical recording medium of the present invention is an optical recording medium of the next generation, such as a BD (blu-ray disk). More particularly, the present invention relates to a write-once read-many optical recording medium, such as a BD-R.
2. Description of the Related Art
Optical recording mediums, namely, optical disks, are classified roughly by read-write system into read-only optical recording mediums, rewritable optical recording mediums and write-once read-many optical recording mediums. The write-once read-many optical disk records data by using the change of the physical property of a recording film that occurs when the recording film is irradiated with a laser beam. The write-once read-many optical disk permits writing information thereto and inhibits erasing recorded information and rewriting. Write-once read-many optical disks, such as CD-Rs, DVD-Rs and DVD+Rs, having such a characteristic are used for preserving document and image files having data that should be protected from dishonest alteration.
Techniques using a short-wavelength laser beam, such as a violet laser beam, for writing and reading optical information have been developed in recent years to achieve high-density recording. Most write-once read-many optical disks to which information is written with a violet laser beam are provided with a recording layer of a thin film of an inorganic material instead of an organic coloring material. Although recording layers of organic coloring matters have been practically applied to conventional optical disks, such as CD-Rs and DVD-Rs, to which information is written with a read laser beam, recording layers of organic coloring matters are inferior in light resistance because those recording layers react comparatively sensitively to violet laser bean. Therefore, recording layers of organic coloring matters have problems in stably preserving recorded signals for a long time.
Recording methods using a laser beam are roughly classified into phase-change recording methods that change the phase of a recording layer, interlayer reaction recording methods that cause plural recording layers interact, and pitting recording methods that form local recording marks. The phase-change recording method needs a reflecting layer and a dielectric layer in addition to a recording layer to enhance the reflectivity and modulation factor of the optical disk. The inter layer reaction method needs plural recording layers. Thus, the phase-change recording method and the interlayer reaction method need an increased number of layers.
The pitting recording method is applicable to an optical disk having a small number of layers because the recording layer of the optical disk has a high reflectivity and can achieve a high modulation factor. Such an optical disk is advantageous in cost and productivity. A recording layer to which information is written by the pitting recording method
As mentioned in JP-A2002-225433, U.S. Pat. App. No. 2004/0241376, WO2007/046390, and JP-A 2007-230207, a recording layer to which information is written by the pitting recording method is made of a metal having a low melting point containing Sn or In as a principal component. For example, a recording layer proposed in JP-A2002-225433 contains Sn as a principal component and at least one of elements in groups 3B, 4B and 5B. A recording layer proposed in U.S. Pat. App. No. 2004/0241376 is made of A(1−y)My, where A is Si or Sn, M is Al or such, y is between 0.02 and 0.8. The applicant of the present invention proposed in WO2007/046390 a recording layer A of a Sn-base alloy containing at least one of Nd, Gd and La, a recording layer B of an Sn-base alloy containing B and may contain at least one of In, Y, La, Nd and Gd, a recording layer C of a Sn-base alloy containing In and/or Co, and at least one of In, Bi and Zn and may contain a rare earth element, a recording layer D of a Sn-base alloy containing a rare earth element and may contain In and/or Bi, and a recording layer E of a Sn-base alloy containing elements in groups 4a, 5a, 6a and 7a and at least one of Pt, Dy, Sm and Ce and may contain Nd an/or Y. The applicant of the present invention proposed in JP-A 2007-230207 a recording layer of an In-base alloy containing a rare earth element, Pd, Co, Pt, V, Ni and Au.
An optical disk of basic construction has a transparent substrate called a base and having a surface provided with concentric circular grooves or spiral grooves called tracks, a recording layer formed on the surface of the substrate, and a cover layer, namely, an optically transparent layer, formed on the recording layer. A laser beam is focused on flat parts of the grooves or lands forming the tracks to write information to or to read information from the optical disk.
FIG. 1 is an enlarged, fragmentary perspective view of an optical disk, which corresponds to FIG. 5 in JP-A H10-334512. As shown in FIG. 1, guide grooves 4 are formed in a substrate 7 to guide accurately a spot 3 of a laser beam (not shown) projected from below the substrate 7 to desired positions. The reflectivity of parts of a recording layer 1 irradiated with the laser beam changes to form recording marks 2. Each of lands 5 is formed between the adjacent guide grooves 4.
Tracking control is performed to make a read-write light beam follow the guide groove. Tracking errors are detected by a push-pull method. Referring to FIG. 2, the push-pull method focuses light reflected from an optical disk on right and left optical detectors separated by a space parallel to the groove and detects the difference between light intensities detected by the right and the left optical detector as a tracking error (tracking signal). The tracking signal is zero when the spot of the read-write light beam is on the groove or the land and is positive or negative when the beam of the read-write light beam is between the groove and the land due to the phase difference between the light reflected from the groove and the light reflected from the land. The thus measured values are used for the feedback control of the position of the spot of the light beam to eliminate tracking error.
Generally, the phase-change optical disk has a low reflectivity and a low modulation factor. Therefore, satisfactory tracking signals cannot be obtained by the push-pull method and stable tracking control cannot be achieved. Methods capable of achieving tracking control by the push-pull method by properly controlling the width or depth of the grooves are proposed in JP-A H7-85503 and JPA H10-334512.
Problems similar to those in the phase-change optical disk reside also in pitting recording optical disks.